Purification of crude terephthalic acid by hydrogenation over a suitable catalyst is well-known. Hydrogenation offers the easiest route for removal of 4-carboxybenzaldehyde (4-CBA) impurity from the crude terephthalic acid (TA). This invention is directed to an improved process for the hydrogenation of crude terephthalic acid in the presence of a catalyst prepared by utilizing palladium metal deposited upon an active carbon support from an organic solution of a palladium salt which reacts with the carbon to produce a catalyst of improved activity and/or selectively in hydrogenating 4-carboxybenzaldehyde to toluic acid.
Catalysts comprising a Group VIII metal of the Periodic Table of Elements upon an inert carrier are known for use in various hydrogenation reactions. They are usually prepared by impregnating a support material with a solution of a compound of a Group VIII metal and reducing the impregnated compound to the metal. Catalyst improvements typically have been directed to obtaining increased hydrogenation activity rather than increased activity and/or selectivity in hydrogenating specific compounds.
It is an object of the instant invention to provide an improved method for preparing a catalyst compound of a Group VIII metal. A particular object is to provide a method for preparing such catalysts having increased catalytic activity and/or selectivity in reduction of 4-carboxybenzaldehyde. Another object is to provide a catalytic composition which comprises crystallites of catalytically-active palladium upon the surface of a porous support material wherein a catalyst of improved activity and/or selectivity is obtained for use in a process for reduction of 4-carboxybenzaldhyde in purification of crude terephthalic acid containing up to 10,000 ppm of 4-carboxybenzaldehyde. Still further objects will be apparent from the following specification.